Production of calcium phosphate



R. MILLER sept. 11, 1951 '3F' CALCIUM PHOSPHATE PRODUCTION 3 Sheets-Sheet l Filed Jan. .'28, 1948 IN V EN TOR. b Y/f /er Sepf- 11, 1951 R. MILLER PRODUCTION OF CALCIUM PHOSPHATE Filed Jan. 28, 1948 3 Sheets-Sheet 2 Sept. 11, 1951 R. MILLER PRODUCTION OF CALCIUM PHOSPHATE 3 Sheets-Sheet 3 Filed Jan. 28, 1948 Patented Sept. 11, 1951 PRODUCTION OF CALCIUM PHOSPHATE Ralph Miller, Woodside, N.v Y., assignor to The Chemical Foundation, Incorporated, a corporation of New York Application January 28, 1948, Serial No. 4,887

This invention is concerned with the economic production of calcium phosphate and particu- `larly with the economic production of dicalcium phosphate and monocalcium phosphate.

Most of the phosphate fertilizer currently produced consists of monocalcium phosphate (CaH4(PO4) 2.H2O)

Since phosphate fertilizer is usually produced from phosphate rock whose composition is approximately '7CaO.2P2O5 (disregarding the other elements usually present in phosphate rock) it can be seen that for every mol of phosphate rock which is converted to monocalcium phosphate, 5 molsV of CaO must be removed or bound up in a non-objectionable form. In the usual practice the excess calcium is removed as, or bound up in the form of, calcium sulfate. If phosphate rock were converted to dicalcium phosphate rather than to monocalcium phosphate, then but 3 Vmols of CaO would have to be removed or bound up for every mol of phosphate rock converted to dicalcium phosphate.

The ease of handling phosphatic fertilizer and its P205 concentration are as important as the availability of its phosphorus content to plant life since so much of the cost of phosphate fertilization is made up of transportation, mixing, bagging and application costs. With respect to most of these items, dicalciurn phosphate offers advantages over monocalcium phosphate. This is well known to the phosphate fertilizer industry.

The potential advantages of producing dicalcium phosphate rather than monocalcium phosphate have been recognized for many years. In fact, the American Chemical Society Monograph Phosphoric Acid, Phosphates and Phosphatic Fertilizers (1927) by Waggaman and Easterwood made this point more than twenty (20) years ago. The subject'is discussed in detail on pages 165 and 166. The matter is sum-med up by these authorities as follows:

While considerable work has been conducted with a view to reducing the quantity of sulfuric acid necessary in producing citrate soluble phosphoric acid without much success the problem does not appear insurmountable and seems well worthy of further research.

A major object of this invention is the pro-4 duction of dicalcium phosphate at an operating Claims. (Cl. 23-109).

2 cost lower than the cost of producing ordinary superphosphate per unit of P205 made available.

An additional object of this invention is the minimizing of the capital investment required to make the P205 in phosphate rock available to plant life simultaneously with increasing the concentration of P205 in the nal product.

A further object of this invention is the production of dicalcium phosphate and monocalcium phosphate substantially free from acid-insoluble constituents using well known chemical engineering unit operations and standard equipment except for being resistant to the corrosive conditions which prevail during some of the steps of the process.

A particular advantage of this invention is that it is operable at temperaturesy which are nominal for neither extremely high nor markedly low temperatures are required.

An additional major advantage of this invention is that it is adapted to substantially continu-- ous operation although the various steps can be carried out batch-wise if desired. There is no need to store or condition an intermediate product as must now be done in making ordinary superphosphate or concentrated superphosphate.

Thel substantial achievement of the above objectives and others, incident to the process, to be pointed out below, represent a marked advance in an art which has undergone no major advances in fundamentals for many years.

The process that constitutes this invention. possesses the following characteristics:

It may be carried out using-sulfuric acid as a reagent whichv is consumed.

Phosphoric acid or phosphorus pentoxide may be employed along withl phosphate rock to produce the desired products. i

Solid monocalcium phosphate is produced as an Vintermediate in the-production of dicalcium phosphate. I

, -A substantially non-volatile solvent for monocalcium phosphate is employed cyclically.

The solution of the monocalcium phosphate in the solvent and its separation from the solvent are readilyv and economically accomplished.

The solvent for monocalcium phosphate aids in the conversion of phosphate rock to monocalcium phosphate.

Phosphoric acid is employed as a cyclic reagent in the production of dicalcium phosphate.

In order to more clearly explain the invention, preferred methods of operation are illustrated in the flow sheet form in the accompanying drawings in which,

Fig. 1 is a liow sheet illustrating the novel operation utilizing an extraneous source of phosphoric acid;

Fig. 2 is a ow sheet illustrating the modied. method in which sulfuric acid is utilized to remove excess calcium;

Fig. 3 is a iiow sheet of a preferred method of operation utilizing refractory phosphate rock as the source material.

The inability of phosphoric acid to convert phosphate rock to solid dicalcium phosphate is well known to the phosphate fertilizer art, This difculty is overcome in the present invention by converting to monocalcium phosphate that portion of the phosphate rock which is ultimately converted to dicalcium phosphate. While it is preferable to convert all the phosphate rock to monocalcium phosphate, in some instances when sulfuric acid is employed, it is more economical to treat a portion of the rock with sulfuric acid in a manner similar to the way phosphate rock is mixed with sulfuric acid in the standard method of making ordinary superphosphate. This variant in the process will be explained later in detail.

It will simplify the description of the process to disregard the constituents of phosphate rock other than calcium, phosphorus and oxygen and to assume that phosphate rock may be designated as 7CaO.2P2O5. One method of carrying out the process follows. The phosphatic raw material such as phosphate rock is dissolved in a circulating solution composed of phosphoric acid containing some dissolved monocalcium phosphate. The dissolution operation is carried out at an elevated temperature preferably in the neighborhood of-lOOo C. The temperature at which the dissolution takes place, as will be appreciated, may be varied widely but if the temperature is too low the process is adversely affected as will later become apparent. The ratio of phosphate rock to phosphoric acid and the concentration of phosphoric acid should be adjusted so that a relatively strong solution of phosphoric acid substantially saturated with monocalcium phosphate at the temperature of dissolution is formed. The P205 concentration of the resulting solution should preferably be substantially in excess of 25% and desirably should approximate 40%. Solutions containing higher concentrations of P205, however, can be formed without causing the process to become inoperative.

The solution formed in the dissolution step is preferably separated from any insoluble particles suspended in it. The solution is then cooled to as low a temperature as is economical. As an example. a temperature of approximately 25 C. is suitable when theA dissolution temperature is about 100 C., the P205 concentration of the resulting solution is about 40% and the solution is substantially saturated with monocalcium phosphate at the dissolution temperature. The solubility of monocalcium phosphate in phosphoric acid solutions increases as the temperature increases. Hence, when the solution saturated with monocalcium phosphate at about 100 C. is cooled, solid monocalcium phosphate crystallizes out of solution. The solid is separated from the cooled solution by settling, ltration or centrifuging or by any convenient combination of these or other applicable liquidsolids separation methods.`

The cooled separated solution is then returned to the rock dissolution step of the process.

The solid monocalcium phosphate separated from this cooled solution has an adherent film of the mother liquor from which it was separated. It is desirable that the solid be contaminated with a minimum of free acid. For this reason it is preferable to separate the crystals from the mother liquor by centrifuging. When solid dicalcium phosphate is the desired product, the solid monocalcium phosphate is hydrolyzed with a limited amount of water at an elevated ternperature to form solid dicalcium phosphate and a phosphoric acid solution containing that p0rtion of the monocalcium phosphate that is not converted to dicalcium. phosphate. The solid dicalcium phosphate is separated from the solution from which it was formed, washed and dried. The separated solution is returned to the cooling step of the process.

Since all the dicalcium phosphate formed in the process is derived from solid monocalcium phosphate, for each mol of product one mol of phosphoric acid must be recycled. The phosphoric acid for recycling in the circuit is formed in the hydrolysis step. Although the extent of the hydrolysis will vary depending upon the conditions prevailing during the hydrolysis step, one mol of phosphoric acid will be formed for each mol of dicalcium phosphate produced. Consequently, the process remains intrinsically in balance regardless of the percentage of conversion.that is achieved. When the process is carried out in the described manner all the CaO contained in the phosphate rock introduced directly into the calcium phosphate production cycle ultimately is converted to dicalcium phosphate. This requires that phosphoric acid or its equivalentv be introduced into the calcium phosphate production cycle in addition to phosphate rock. There are numerous Ways by which this phosphoric acid or its equivalent can be made and introduced into the calcium phosphate production cycle.

Phosphoric acid or its equivalent is made by treating phosphate rock with sulfuric acid by the so-called wet process or by the blast furnace or electric furnace process which'produces elemental phosphorus as an intermediate. In the one case the excess calcium is removed as calcium sulfate; in the other as a calcium silicate slag. The nal product is composed in part of P205 which has been substantially or completely separated from the CaO to which it was bound in the rock before being added to the calcium phosphate production cycle. The following equations represent an idealized version of the reactions in the process when the excess calcium is removed as calcium silicate slag:

It can be seen (reactions 3 and 4) that for every 4 mols of phosphate rock Which are treated and completely converted to dicalcium phosphate, 12 mols of phosphoric acid or its equivabined Iwith .the -2-81mols-.of-A phosphoricxacidfiormed: inf the:hydrolysis..step.7 (reaction 4-) l' to sccllrethe necessary amount 4of-rilvhcisphoric acidrequired-tina reaction 3 to .convert 4-f3molsf. of.- phosphaterockitomonocalcium. phosphate:

"z Theproduction. of the-` 121 mols of: phosphoricacidI asindicated ina reactions 1 and zzmayalsobe made by treatment ofphosphatel rockwith-r sulfuric acidz' Reaction 5 represents` the standard wet process ofjrmaking phosphoric-acid; As is well known the large scale productionof' phosphoric-l acid by the Wet process is not simple; In addition, it requires an elaborate plant- The nal= product is relatively dilute comparedwith-the-concentration of phosphoric acid produced by thermal-methods.

Since; the phosphoric acid-A formed inthe hydrolysis step is constantlybeing recycled and= since-the-solution formed-inthe hydrolysis-step is returned to the crystallization step; l2mo1s-of` phosphoric acidmust be added-to the circulatingl solution for every 4 mols of-'phospha-terock that dissolves completely. Whenthe process is being carried outin this manner, the-12 mols of phosphoric acid may be added-at any convenient' part of the cycle just as longas solid monocalciumphosphate crystallizes; out inthe-cooling step of the process.

The foregoing procedure-is one-that-canbe-einployed when for any-reason itis founddesirableto treat one part of`the phosphate rock toproduce phosphoric-acid or its equivalent andto-employ. the phosphoric acid so-made inthe calcium phosphate production-- cycle. The phosphoric acid, as noted previously, may be made by thermal processes orby thewet process. This method, of procedure requires the substantial total dissolution of the phosphaterock in a phosphoric acid solution. This can be achieved in most instances even if f the phosphate rock tends tobe refractory. Ifsulfuricacid-is employedV to remove theexcess calcium, it is not necessary to produce phosphoric acid as such bythe wet process." Instead it is; preferable tomodify the process. In the modiedf process the: excess calcium isV removed' within the` calcium phosphate production cycle. The modified` process may. berepresented by the following-reactions:-

Reaction 6 represents the. treatment of one molf` calcium phosphate-hydrolyzed to dicalcium phos-v phate, 3 mols of dissolved calcium, which conveniently maybe representedfas: dissolved monocalcium phosphate, is precipitatediwith 31mols of: sulfuric acid to; formsolid calcium sulfateand: 6

mols of phosphoric acid. Reaction-l 8@ representsthe conversion` of.' the4 excess calciumE presen-t as dissolved calcium phosphatetQ-solid calcium-.Sula

6a 'fata The: solid calcium sulfate is read-ily.- removed-from the system by ltration or its equiv.-v alent.- The. phosphoricacid formed concomitant- 1y, with theA formation of the solid-calcium sulfate is:l thenused in the rock dissolution step of the process-.to dissolve the excess C-aO contained-i in the succeeding quantity of phosphate rock that is dissolved. As in the previous process, the phosphate rock is dissolved at. relatively elevated temperatures, i. e. inthe neighborhood of 100 C; and the-solution leaving the dissolution step is relatively strong phosphoric acid substantially saturated.- with monocalcium phosphate at the-.elevated temperature. The hot, saturated solutionl isv cooled to crystallize out solid. monocalcium phosphate. The solid monocalcium phosphate is. This. solid separated from its mother liquor. monocalcium phosphate is then hydrolyzed-"to dicalcium phosphate and a phosphoric acid' soluv tion saturated with monocalcium phosphate. Thev I tional phosphoric acid produced as a result of the formation of the solid calcium sulfate is recycledtothe rock dissolution step of the process 4after being warmed. Thisis the preferred methodo carrying out the process.

Another Way in which the process may be modiii-ed: using sulfuric acid to remove the excess. calcium is useful when particularly refractoryphosphate rock is employed. The process is carried-f outin a manner essentially similar to theprocedure -descri'bed above. IfA the phosphate rockis-refractory, it Will be dilcult to dissolve all ofits Pzoscontent completely in phosphoric acid. To recover the diiiiculty soluble P205 content of the-rock-therefractory portion is treated directlywith sulfuricacid using-sufficient` acid to convert all the P205 content to a form that is soluble in. phosphoric acid. The treatment of the refractory portiony ofthe rock with sulfuric-acid constitutes-only-a, simple mixing operation. It can be. carried out in about the same Way that ordinaryy This produces a mix.-

superphosphate is made. tura composed principally of calcium sulfate and. monocalciumphosphate. The resulting mixture is then leached With the circulating solvent solu.- tion, at any convenient part of the cycle. The P205 content of the mixture dissolves in the cir-- culating solution while the calcium sulfate doesv not. Obviously, any part. of the sulfuric acid. used'. tov treat the refractory portion ofthe rock is deducted from the acid employed to remove- -ample sulfuric acidl to convert all-the P205- in theundissolyed port-ion of the rock. to a form soluble;

in-.ph0Snhpric-acd- From the foregoing, the essential concepts em-` bodied in thevnovel process will be appreciated. It can be seen that thev non-Volatile solvent for monocalcium phosphate is a strong phosphoric acid solution. The solution of the'monocalcium phosphate in strong phosphoric acid takes place at a high temperature point of the cycle and the separation of the monocalcium phosphate takes place atthe low temperature point of the cycle.

Since the solute is very soluble in the solvent at` elevated temperatures and is but moderately soluble at the low temperature point of the cycle, the amount of solute which may be transferred per unit of solvent circulated is relatively high. For example, 100 pounds of a 48% phosphoric acid solution can dissolve in excess of 33 pounds of monocalcium phosphate at 100 C. At 25 C., the same quantity of 48% phosphoric acid solution can dissolve about 15 pounds of monocalcium phosphate. Thus about 18 pounds of monocalcium phosphate can be separated per 115 pounds of solution circulated. This is a very favorable ratio of material transferred per pound of solution circulated. It is apparent that the process can be so operated that the heat required to warm the solution up to the dissolution ternperature from the crystallization temperature is the principal heat burden of the process.

It is well known that among other factors the conversion of phosphate rock to monocalcium phosphate by reaction with phosphoric acid varies with the concentration of the acid employed and the ratio of P205 to Ca in the mixture and that the most important variable is the P205 to CaO ratio. Since in the present process the solvent for-the monocalcium phosphate which is formed is phosphoric acid, the P205 to Ca0 ratio is very much higher than in any other process now employed. Fortuitously, then, phosphoric acid possesses not only useful solvent properties for monocalcium phosphate but it also is particularly helpful in the formation of monocalcium phosphate. The role of phosphoric acid as a cyclic reagent is to aid in the conversion of the phosphate rock to monocalcium phosphate. If phosphoric acid was not employed as a cyclic reagent in the manner described, then the process would become inoperative since the conditions by which it is possible to convert phosphate rock to dicalcium phosphate employing the stoichiometric quantity of phosphoric acid are not known to date, and may be non-existent. The phosphoric acid is recovered for recycling by the hydrolysis step of the process.

Figure 1 illustrates diagrammatically one method in which the process may be carried out when an extraneous source of phosphoric acid is used. This is an idealized version of the process.

When the process is carried out in practice@ with the principles of the invention, as previously l explained the rock is contacted in the digester with a special solution. This comprises a phosphoric acid solution of about 50% concentration, more or less, which contains sufficient dissolved monocalcium phosphate to be less than half saturated at 100 C. The initial make up of phosphoric acid plus any necessary replacement supply is drawn from the acid storage 3 and is passed through lineal! into line 5 which constitutes a portion of the continuous circuit or cycle utilized in the invention. The solution in line 5 which, as noted, is a strong phosphoric acid solution 'containing some dissolved monocalcium phosphate, is heated in a suitable heating unit B up to the desired temperature which preferably is substantially 100 C. and discharged into the digester through line 1. As shown in the digester the rock passes countercurrently tothe acid. The conditions thus established in the digester favor a rapid reaction of the acid with the rock. While the acid does contain some dissolved monocalcium phosphate it is not saturated and the acid employed is a strong acid. Under thev conditions` of the operation the temperature is high and there is a large excess of phosphoric acid over that requiredto convert all of the rock to monocaloiumphosphate. As is known, the extent of the reaction between the rock and acid depends upon the ratio of P205 to CaO and upon the concentration of the acid. The ratio of P205 to Ca0` vemployed in the present process is larger than in any process now employed. The concentration of acid is sufliciently high so that it readily reacts with the rock. As is known the reaction between the acid and rock is exothermic and hence there is no diiiiculty in keeping the solution at a sufficiently elevated temperature to insure rapid reaction with the rock.

The digester 2 may be of any suitable type of construction provided with an acid resistant lining. For example, this reaction stage may comprise one or more digesters lined with carbon brick.

The hot phosphoric acid solution now substantially saturated with monocalcium phosphate flows from the upper section of the digester and is treated in a manner to be described to remove a quantity of the contained monocalcium phosphate and the acid solution containing some dissolved monocalcium phosphate is recycled.

In' the digester the acidinsoluble constituents of the rock are not dissolved by the acid solution.- These accumulate `in the bottom section of the digester and are removed at convenient periods through the solids discharge line 8. The discharged solids are washed in the washer 9 with hot water entering line I0 to recover the adherent acid solution. The solids are discharged from the system through line II. The wash liquor passes through line I2 and is added to the recycle 'solution in line 5.

The hot saturated solution of phosphoric acid is withdrawn from the 'digester through line I3 and is passed to a stage I4 wherein it is cooled to an extent suicientto crystallize a substantial portion of the dissolved monocalcium phosphate. Whenever required or whenever deemed desirable the solution passing through line I3 may be ltered, centrifuged or otherwise treated to lremove suspended or entrained solids before discharge to rthe cooling stage.

The crystallizing stage may be of any desiredr type. Preferably this comprises a unit, such as concentration concomitantly withv cooling.

` In this manner, part or all of the water introduced into the circuit or system is removed advantageously utilizing the specific heat of the hot eiliuent'from the digester.

' i .The cooled slurry :formedinthe Icrystalliz'er l! lowsithrough'line I6 .to .a suitable liquid-solids separation stage .such .for -.example .las the .cene trifuge I 1. flhe solids separated inftheicentrifuge consistingessentially of monocalcium phospate are discharged .through .line I8 .and the :acid liquor .through .line I9. .The .discharged :solids may be segregated vand -storedforany subsequent treatment or. disposition. :In the preferred. methodl of.` producing dicalcium. .phosphate such solids are passed .directly to a 1hydrolyzerz2l! tfwherein a portion of .themonocalcium .salt islconverted to dicalcium..phosphatc anda phosphoricacid solution. The Water utilized.forhydrolysislmay conveniently be .introduced .into .the .ahydrolyzer through linel .2 I Since .the .ex-tent. of .hydrolysis increases with increasing temperature. it preferableltoelect .such hydrolysisfiatias high altem: perature as .is l feasible.. The vhydrc'alysisI can :he carried out at .about 10.090. Without unduedifculty. By operating; the hydrolyzerunder. pressure .more .elevated .temperatures -may .be .employed withconcomitant reduction. in theamount of =Water required per .pound f.dicalcium.phos phate .formed .The .unconverted .mcnocalcium phosphatednsolution.in .phosphoricacid is dischargedthrough 1ne..22 andpasses back toiline 3 fordischarge into the-crystallizationstage. i .The solid :dicalcium phosphate .formed-inthe hydrolyzer .is passed ...through .line 23 .toca separation .stage `such .as .the ...centrifuge 24. ,The aqueous effluent from the centrifuge .passes through.line 25 .back `to the hydrolyzer. The solid .dicalcium .phosphate y.leaves the .centrifuge through .conduit `2.6 and is discharged into .the dryer .21. The dried .product '.s l.Witlfidr-awn .at 28'..to storage or shipment.

vReferring :to the centrifuge/.I1 it-willbe .seen thai-the solution of .monocalcium phosphate .in phosphoric .acid .passes through line I9. .for .reemployment rin the system. .This Y:solution .is heated inithe heating -stage .and is .fed..to :the digester. .As is .'shown,.in' transit to Lthedigester the vrecycle .solution .is fortified. by -..admixture with Wash. liquor from .washerf andsany .necessary replenishment :from .phosphor-ic vacid .-supply :3.

Figure V12 illustrates. how .the process #may -f be carried .out .when sulfuric acid ,is employedlto remove theeexcess :calcium .in `the rock-.ainsteadflof employing `an extraneous source `of -phosphoric acid. Thedetails of:.the..process are .essentially similar to those setiorthlinthe process .depicted in Fig. 1 except for `the..precipitation.ofthe Vexcess calcium as Ycalcium sulfate by .meansofsulfuric acid.

As shown in.Fig. phosphate rock :fromcsupply 3l is .charged .to thexdigesterl32.andrasdescribed is contacted therewitha Ahot .phosphoric acid solution containing some .monocalcium phosphate introducedthrough line V33 whichnhas been heated in heater Y3.4,. The insolublematerial accumulating in the digester is passed through line v35 for washing in vWasher `36 .by means of wash `.Water introduced through line-S'I. The Washed solids are discarded throughline 38 and the Wash liquor is introduced into theacid recycle circuit through line 39.

As in the .process of Fig. 1 vthe hot eflluent acid liquor passes from .the digesterthrough line.-4.0 to the lcrystallizing stage 4l vfrom which water may be removed as vapor through 1ine42. The slurry formed in the crystallizer passes through line-,43 to the centrifuge. for lseparationof the solid and liquid constituents. .The separated 10 solids-are passed through 1ine45. andthelquid eluent through Yline-.116. The solids are Adischarged into the '.hydrolyzer .M and are hydrolyzed in fthe'manner .described by Water intro: duced through linezll. :Theacid liquor from the hydrolyzer .passes by Wayof line 149 back to :the crystallization stage. VThe Wet solids are dischargedthrough line 150 to '.thecentrifuge 5 I in which they .are dewatered. The liquid from `the centrifuge is recycled .to the'hydrolyzer through line.52 while .the dewatered -solids are passed through line 53 into the .dryer `54 and aredis.-

charged after beingdried through line; 5.

.The acidssolution. separated .in .the centrifuge 44 .is .treated .to .remove .the .excess calcium which tends to .buildup in the circuit. As shown,.this solution is passed through line 4.6 to reactor :.56 wherein it is .contacted with .sulfuric .acidadf mitted through line :5l to precipitate the excess calcium. as` solid calciumsulphate. YThe resulting slurry-is.dischargedthrough line 58 to..the:.cen trifugeb59. .The .separated acid liquoris. recycled to-.the.digestion stage .through .line .60 as shown. The wet. solidcalciumcsulfate is .discharged from thecentrifuge .through line 5I and is Washed in the unit. 62 by Water .admitted .through .1in er,63, 'Thelwashed .solids aredischarged .throughline 6.4 while the acidenriched washliquor is introduced into the yrecycle circuit through. line 65.

;In\Fig. .3 there.. is illustrated apreferred modif cation .of -the .processwherein the phosphate rock. employedis :particularly .refractory. It fwill becbservedirom an .inspection of the'flow sheet that .it is essentially .similar .to that :fshown f in Figi, with the vadditional showing that the;solv ids v.from thie digester, .containing recoverable phosphatevalues-.are treated L.vvith sulphuric acid toi.produce phosphate compounds solubleA in uphosgphoricacid.

The `.general circuitl ow nof materials through thasystem .is as .has Vbeen previously described. Rocksfrom storage JI .passesto `.digester 12. and is reacted .with-A the phosphoric acid :solutirmngenter ing.line.1.3..and1which has ypreviously been heated infheater 14.

. 'I'he;.-solids.=accumulating in :thebottom ofthe digester are f. passedlthrough line 15 and are re.- actedin reactor 'I6 with sulphuric acid admitted through line .17. The reaction. products from ree actor 21.6 are .passed through. line 18..to aileaching Vessel 1:9.wherein thesolidsare leachedv with the hot eiliuent acidsolution from :.thedigester :ad-A mitted .through line .80. .The enrichedhphosphoric acid-.solution passesthrough line 8 I 1. and is then treated in .the same'manner. ashas been .prevousf ly .described .with respect to .the solution :.dise charged .through .lineJI .of Fig. .2.

The leached -solids are discharged @from the gleachingvesseLIS. througlrthelineZ to thewashe er 83 .and v.are .Washed with water admitted throughline 3.4. The :Wash liquoris .discharged through line .into `linefBI .and lpassessisdescribed to-the..crystallization stage.

.As noted above, the acid vliquor .'.ilowin-g through line.8l `is treated invexactly'thecsame manner as the .equivalentliquor chargedthrough line 1.40 of rFig. y2. Thisliquorpasses to crystal-.- lizer. 81 from'which Water; inthe. form of .steam mayv be -evolved and discharged through linew. The slurry formed in the crystallizer is;.passe,d through line 89Mto the centrifuge ,-90. The Wet solid monocalcium phosphate -.is .discharged through line 9| and the solution through linef?. 'Il-1e monocalcumfphosphate is admitted tofhydrolyzer ..93 wherein .it hydrolyzed-with water admitted through line 94. The liquid from hydrolyzer 93 comprised of a solution of phosphoric acid .containing some monocalcium phosphate enters line 8l and is passed to the crystallizer 81. The dicalcium phosphate formed in the hydrolyzer passes through line 9B and is dewatered in the centrifuge 91. The water separated in the centrifuge is recycled through line 98 to the hydrolyzer. The dewatered phosphate passes through line 99 and is dried in dryer |00 from which it is discharged through line |01 to storage or for transportation.

The production of monocalcium phosphate, as indicated, is also included within the scope of this invention. When monocalcium phosphate is produced, the consumption of sulfuric acid is increased since morecalcium must be removed than when dicalcium phosphate is produced. If an extraneous source of phosphoric acid is employed, then the amount of phosphoric acid required per mol of phosphate rock employed in the monocalcium phosphate production cycle is increased. The hydrolysis step in the dicalcium phosphate process can be eliminated provided the phosphoric acid coating the monocalcium phosphate crystal is not seriously objectionable'. If the phosphoric acid coating the crystals is objectionable, it is necessary to eliminate it. It is not possible to remove the excess phosphoric acid from the crystals by washing them with water. To produce monocalcium phosphate free from excess phosphoric acid and free from acid insoluble constituents, a portion of the monocalcium crystals is hydrolyzed with water as in the dicalcium phosphate process to form solid dicalcium phosphate and a solution of phosphoric acid containing some dissolved calcium phosphate. The solid dicalcium phosphate is then intimately mixed with the monocalcium phosphate crystals containing excess phosphoric acid. The proportion of dicalcium phosphate to monocalcium phosphate is adjusted so that a m01 of dicalcium phosphate is added for each mol of free phosphoric acid adhering to the monocalcium phosphate crystals. The free phosphoric acid reacts with the solid dicalcium phosphate to form monocalcium phosphate. A small excess of dicalcium phosphate can be used to improve the handling properties of the mixture. Any excess water can be removed by drying- In this Way it is possible to produce a product whose composition can be varied at will.

Relatively pure phosphoric acid, monocalcium phosphate and dicalcium phosphate can be made economically by integrating the production of these products along with the production of fertilizer grade calcium phosphate. Practically all phosphate rock contains appreciable quantities of combined iiuorine, iron and aluminum as well as silica. The dissolution of phosphate rock in phosphoric acid causes significant amounts of these impurities to dissolve along with the lime and P205. As the solvent solution is recycled, these impurities tend to build up in the circulating solution. That portion of the iron and aluminum in the rock which is converted to the phosphate leaves the system along with the acidinsoluble constituents of the rock and in the final product. Fluorine that dissolves is vaporized in the rock dissolution step, the rewarming step and in the vacuum crystallizer; the remainder leaves the system along with the calcium phosphate.

Pure phosphoric acid is made by acidulating pure calcium phosphate either the mono or the 12 di with an equivalent amount of pure sulfuric acid. Solid calcium sulfate is precipitated and phosphoric acid containing only minor amounts of dissolved calcium and sulfate is formed. The solid calcium sulfate is separated from the pure phosphoric acid. The pure crystalline calcium phosphate is produced by recrystallization from phosphoric acid solutions. The impure acid plus its calcium phosphate content which results from the recrystallization operations are added to the fertilizer calcium phosphate production cycle. In this way there are no losses resulting from values contained in solutions which otherwise might have to bediscarded.

Since, as has been explained, it is essential that solid monocalcium phosphate crystallize from the circulation solution at the lowest temperature to which the circulation solution is reduced, the phosphoric acid concentration of the circulation solution must be suiciently high so that the solid phase in equilibrium with it at the reduced temperature is monocalcium phosphate. Therefore, the minimum phosphoric acid concentration of the circulation solution employed in this process depends upon the lowest temperature to which the circulation solution is reduced. The expression strong phosphoric acid as used in this specication and claims means a phosphoric acid solution that is in equilibrium with solid monocalcium phosphate at the lowest temperature to which the circulation solution is subjected.

From the foregoing description it will be appreciated that the novel concepts of the invention may be embodied in a number of specically diiferent processes, all of which are as eminently simple as they are technically effective. These several modiiications have been described to illustrate the Wide permissive variation in operative technique to adapt the novel process to different types of raw material and toobtain specifically different valuable end products. In all such described and equivalent modifications the fundamental inventive concepts are invoked, namely, the establishment of a circulation fluid, or recycle solvent, comprised of strong phosphoric acid having some dissolved monocalcium phosphate; iiowing this in a closed cycle or circuit; enriching the solution in calcium phosphate in some chosen section of the cycle; removing phosphatic values in another sectionV of the circuit by the simple expedient of reducing the temperature, preferably coupled with the simple conversion of such phosphatic values to a more desirable product with concomitant regeneration of the necessary cyclic reagent required to convert the phosphatic source material to a form soluble in the circulation fluid. As explained previously the excess calcium which tends to build up in the system must be removed. This may be done, as pointed out, either by removing the calcium as an insoluble, readilyl separable salt, or by introducing phosphoric acid in the circulation liquid. In either event the circulation fluid pro-vides a most amenable vehicle for such treatments.

I claim:

1. A method of producingl dicalcium phosphate which comprises establishing a solution of strong phosphoric acid containing some dissolved monocalcium phosphate, flowing such solution in a closed circuit; contacting the solution at an elevated temperature with phosphate rock in-one section of the circuit underV conditions controlled to dissolve the rock to therebyv increase Athe amount fimonnfcalcium phosphate 'dissiveu' in the solution; flowing vthe monocalcium phosficiently to crystallizefa portion of the dissolved monocalcium phosphate, separating the solid monocalcium phosphate from the-solution, hydrolyzing'the separated solid monocalcium phosphate to dicalcium phosphate and phosphoric acid and further contacting the monocalcium phosphate-depleted solution with phosphateirock in thecircuit andintroducing the phosphoric acid produced-in thehydrolyzing'step into the said'circuit.

2. A method of .producing dicalcium phosphate which comprises establishing a solution of strong phosphoric acid containing some dissolved monocalcium phosphate; flowing such solution in a vclosed circuit, contacting theV solution-at an -elevated temperature with phosphate rock in one section of the circuit under conditions controlled to dissolve the rock to thereby increase the amount of monocalcium phosphate dissolved in tion to another section f the circuit andthere reducing'the temperature sufliciently to crystalliz`e 'a portion ofl the dissolved monocalcium phosphate, separating -the solid Imonocalcium .phosphate Vfrom the :monocalcium phosphatedepleted solution, -hydrolyzing the separated solid monocalcium phosphate with water to form solid dicalcium phosphate and a solution of phosphoric acid containing some dissolved monocalcium phosphate, separating the solid dicalcium phosphate from its associated solution and further contacting the last said solution and the monocalcium phosphate-depleted solution with phosphate rock in the circuit.

3. The process of claim 2 in which the phosphate rock is contacted with the strong phosphoric acid solution containing some dissolved Vtion of the dissolved monocalcium phosphate,

separating the solid monocalcium phosphate from the monocalcium phosphate-depleted solution; hydrolyzing the separated solid monocalcium phosphate with Water to form solid dicalcium and a solution of phosphoric acid containing some dissolved monocalcium phosphate, further contacting the last said solution and the monocalcium phosphate-depleted solution with phosphate rock in the circuit and introducing additional amounts of phosphoric acid in the circuit.

5. The process of claim 4 in which the phosphate rock is contacted with the strong phosphoric acid solution containing some dissolved monocalcium phosphate at a temperature between about '75 C. and 125 C'.

6. A method of producing dicalcium phosphate which comprises establishing a solution of .strong phosphoric acid containing some dis- 14 solved monocalcium phosphate, wing such solution in a closed circuit; contacting the solution-atan elevated temperature with phosphate rock in one section of the circuit under rconditions controlled to'dissolve the rock to thereby increasethe amountof monocalcium phosphate 'dissolved Vin the solution; flowingthe monocal- -fcium phosphate-enriched solution to another section of the circuit and there reducing the temperature suiiiciently to crystallize a portion of -thefdissolved monocalcium phosphate, separating the solid monocalcium phosphate from the monocalcium phosphate-depleted solution, 'hydrolyzingthe separated solid monocalcium phosphate to dicalcium phosphate and phosphoric acid, introducing the phosphoric acid produced in the hydrolyzing vstep into saidcircuit, adding tov the monocalcium phosphate- 'depleted solution an'amount of sulfuricacid substantially equivalent to the difference between theCaO content of the dissolvedfportion ofith'e rock and the CaO` content of the solid dicalcium phosphate formed to thereby form solid calcium 'sulfate' and phosphoric acid, separating the calcium 'sulfate from the resulting solution and `further contacting such solution with phosphate rockin the circuit.

7. A process in vaccordance with'claim 6 in which the separated solid calcium sulfate is washed with water and the wash liquor is added to said resulting solution.

8. A method of producing dicalcium phosphate which comprises establishing a solution of strong phosphoric acid containing some dissolved monocalcium phosphate; lowing such solution in a closed circuit, contacting the solution at an elevated temperature with phosphate rock in one section of the circuit under conditions controlled to dissolve the rock to thereby increase the amount of monocalcium phosphate dissolved in the solution, flowing the said hot, enriched solution to another section of the circuit and there reducing the temperature su'iciently to crystallize a portion of the dissolved monocalcium phosphate, separating the solid monocalcium phosphate from the monocalcium phosphate-depleted solution, hydrolyzing the separated solid monocalcium phosphate With water to form solid dicalcium phosphate and a phosphoric acid solution containing some dissolved monocalcium phosphate, separating the solid dicalcium phosphate from the phosphoric acid solution containing some dissolved monocalcium phosphate, recycling the last said solution to the crystallization step of the process, adding an amount of sulfuric acid to the monocalcium phosphate-depleted solution substantially equivalent to the difference between the CaO content of the dissolved rock and the CaO content of the solid dicalcium phosphate formed in the hydrolysis step to form solid calcium sulfate and phosphoric acid in situ in the monocalcium phosphate-depleted solution, separating the solid calcium sulfate from the associated solution and further contacting the said separated associated solution with phosphate rock in the circuit.

9. A method of producing dicalcium phosphate which comprises establishing a solution of strong phosphoric acid containing some dissolved monocalcium phosphate, owing such solution in a closed circuit, contacting the solution at an elevated temperature with phosphate rock in one section of the circuit under conditions controlled to dissolve a portion of the rock to thereby in- ,crease the amount of monocalcium phosphate dissolved in the phosphoric acid solution; separating the thus enriched phosphoric acid solution from the undissolved portion of the rock, then intimately mixing the undissolved portion of the rock with suilicient sulfuric acid to form solid calcium sulfate and to convert the residual P205 content of the rock to a phosphate compound soluble in a phosphoric acid-containing solution, contacting the reaction mixture with a phosphoric acid-containing solution to thereby dissolve the phosphoric acid-soluble components of the mixture, introducing the resulting solution into the circuit; flowing the phosphoric acid solution enriched with monocalcium phosphate resulting from the phosphate rock contacting step to another section of the circuit and there reducing the temperature of the solution suciently to crystallize a portion of the .dissolved monocalcium phosphate, separating the solid monocalcium phosphate from the monocalcium phosphate-depleted solution; hydrolyzing the separated solid monocalcium phosphate with Water to form solid dicalcium phosphate and a phosphoric acid solution containing dissolved monocalcium phosphate, separating the solid dicalcium phosphate from its associated solution; recycling the separated phosphoric acid solution containing dissolved monocalcium phosphate to the crystallization step of the process and further,

contacting the monocalcium phosphate-depleted solution With phosphate rock in the circuit.

'10. The process of claim 9 in which sulfuric acid is added to the monocalcium phosphate-depleted solution to form solid calcium sulfate and phosphoric acid in situ in the monocalcium phosphate-depleted solution and separating the solid calcium sulfate from the solution with which it was associated, the amount of sulfuric acid being determined by subtracting rom an amount of sulfuric acid equivalent to the CaO in the rock which dissolves in the circulating solution the total of an amount of sulfuric acid equivalent to the CaO contained in the solid dicalcium phosphate product plus the sulfuric acid intimately mixed with the undissolved portion of the rock.

RALPH MILLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,459,124 Webster June 19, 1923 2,021,527 Suchy et a1 Nov. 19, 1935 2,057,956 Kaselitz Oct. 20, 1936 2,343,456 Henninger Mar. 7, 1944 GTHER REFERENCES Elmore et al., Industrial 8: Engineering Chemistry. vol. 32, No. 4 (1940) pages 580-6.- 

1. A METHOD OF PRODUCING DICALCIUM PHOSPHATE WHICH COMPRISES ESTABLISHING A SOLUTION OF STRONG PHOSPHORIC ACID CONTAINING SOME DISSOLVED MONOCALCIUM PHOSPHATE, FLOWING SUCH SOLUTION IN A CLOSED CIRCUIT; CONTACTING THE SOLUTION AT AN ELEVATED TEMPERATURE WITH PHOSPHATE ROCK IN ONE SECTION OF THE CIRCUIT UNDER CONDITIONS CONTROLLED TO DISSOLVE THE ROCK TO THEREBY INCREASE THE AMOUNT OF MONOCALCIUM PHOSPHATE DISSOLVED IN THE SOLUTION; FLOWING THE MONOCALCIUM PHOSPHATE-ENRICHED SOLUTION TO ANOTHER SECTION OF THE CIRCUIT AND THERE REDUCING THE TEMPERATURES SUFFICIENTLY TO CRYSTALLIZE A PORTION OF THE DISSOLVED MONOCALCIUM PHOSPHATE, SEPARATING THE SOLID MONOCALCIUM PHOSPHATE FROM THE SOLUTION, HYDROLYZING THE SEPARATED SOLID MONOCALCIUM PHOSPHATE TO DICALCIUM PHOSPHATE AND PHOSPHORIC ACID AND FURTHER CONTACTING THE MONOCALCIUM PHOSPHATE-DEPLETED SOLUTION WITH PHOSPHATE ROCK IN THE CIRCUIT AND INTRODUCING THE PHOSPHORIC ACID PRODUCED IN THE HYDROLYZING STEP INTO THE SAID CIRCUIT. 